Method of reconstructing an analog signal, particularly in digital telephony applications, and a circuit device implementing the method

Abstract

A method of reconstructing an analog signal, particularly for digital telephony, comprises a first step of digital-to-analog conversion, wherein a first reconstruction of the analog signal is provided by introducing a distortion component into the frequency spectrum whose amplitude decreases with the signal frequency, and a second step of filtering carried out by means of a reconstruction filter provided with integrators and having a cut-off frequency F t . That attenuating distortion component is utilized instead of one of the integrators in the reconstruction filter, to afford a reduction of the overall design of the circuit device operating in accordance with this method, and bring about, as a result, decreased occupation of the integrated circuit and power dissipation.

I claim: 
     
       1. A method of reconstructing an analog signal previously converted by pulse code modulation to a digital signal, comprising the steps: a. subjecting the received digital signal to a digital-to-analog conversion process to produce a first reconstruction of the analog signal, said digital-to-analog conversion process introducing into the frequency spectrum of the first reconstruction a distortion component whose amplitude decreases with increasing frequency,   b. subjecting the first reconstruction of the analog signal to low-pass filtering by means of a reconstruction filter including only four integrators with a cut-off frequency F t  to produce a second reconstruction of the analog signal, said four integrators present in said reconstruction filter functioning to remove the distortion component of the first reconstruction, the frequency response of the introduced amplitude distortion component functioning to attenuate the signal above the cut-off frequency F t .   
     
     
       2. A method as set forth in claim 1, wherein the reconstruction filter has a passband of 0-3300 H z , where F t  =3300 H z , said digital signal being a digital telephonic signal. 
     
     
       3. The method of claim 2, wherein said first reconstruction has a frequency response defined by: ##EQU2## where fs equals the sampling frequency used in the digital-to-analog conversion process and f equals any frequency of the analog signal, and each integrator has a transfer function substantially equal to: ##EQU3## 
     
     
       4. A circuit for reconstructing an analog signal previously converted by pulse code modulation to a digital signal in digital telephony, comprising: a digital-to-analog converter having an input for receiving the digital signal and producing at an output a first reconstruction of the analog signal, said digital-to-analog converter introducing into the frequency spectrum of the first reconstruction a distortion component whose amplitude decreases with increasing frequency,   a low-pass reconstruction filter having an input for receiving the analog signal from the converter, said reconstruction filter having only four integrators with a cut-off frequency F t  producing at an output a second reconstruction of the analog signal, the integrators present in said reconstruction filter functioning to remove the distortion component in the first reconstruction, the frequency response of the introduced amplitude distortion component functioning to attenuate the signal above the cut-off frequency F t .   
     
     
       5. A circuit as set forth in claim 4, wherein the reconstruction filter has a passband of 0-3300 H z , where F t  =3300 H z . 
     
     
       6. The circuit of claim 5, wherein said first reconstruction has a frequency response defined by: ##EQU4## where fs equals the sampling frequency used in the digital-to-analog converter, and f equals any frequency of the analog signal, and each integrator has a transfer function substantially equal to: ##EQU5## 
     
     
       7. The circuit of claim 5, wherein the final stage of said filter comprises a power operational amplifier. 
     
     
       8. The circuit of claim 4, wherein the first reconstruction has a frequency spectrum corresponding to the curve 7 in FIG. 2 of the drawing.